sourcemap: return a sourcemap along with the CSS output. Using the source
option of css.parse is strongly recommended when creating a source map.
Specify sourcemap: 'generator' to return the SourceMapGenerator object
instead of serializing the source map.

inputSourcemaps: (enabled by default, specify false to disable) reads any
source maps referenced by the input files when generating the output source
map. When enabled, file system access may be required for reading the
referenced source maps.